Thermistors malfunctioning

I broke the leads on my original thermistor when changing the nozzle. I tried to solder it, but had no luck.

I bought a three pack of thermistors from IAP last week, and plugged a new one and was good to go. One 12 hour print later, my thermistor is not reading any temps. I jiggled the wires a bit to see if I could reconnect a broken wire, but never had success. I plugged another thermistor in and instantly got a room temp reading, so it's just the thermistor.

I reassembled the extruder with the new thermistor, but now I'm wondering how long will that one last...

What temp is too high for the thermistors? I've been running my hotend at ~240 which seems like it's within normal range for the extruder.

Aside from not banging into it with a wrench when I'm replacing the hotend (I've learned that lesson) what else do I need to do to baby the thermistor?